Method for production of paper

ABSTRACT

The present invention relates to a method for the production of paper from a suspension containing cellulosic fibers comprising adding to the suspension and/or to a formed paper prepared from the cellulosic suspension i) a nitrogen-containing organic compound separately and/or in conjunction with, ii) an aqueous dispersion of an acid anhydride and iii) at least one shading dye, draining the suspension on a wire to form a paper.

The present invention refers to papermaking and more specifically to amethod for the production of paper wherein a nitrogen-containing organiccompound, an aqueous dispersion of an acid anhydride and at least oneshading dye are added to a cellulosic suspension.

BACKGROUND

In the papermaking art, an aqueous suspension containing cellulosicfibers, and optional fillers and additives, referred to as stock, is fedinto a headbox which ejects the stock onto a forming wire. Water isdrained from the stock through the forming wire so that a web of paperis formed on the wire, and the web is further dewatered and dried in thedrying section of the paper machine. The obtained water, usuallyreferred to as white water and containing fine particles such as finefibers, fillers and additives, is usually recycled in the papermakingprocess. When producing paper where the aesthetic impression, includingproperties such as whiteness, brightness and/or the printability isimportant, hereinafter referred to as higher grade paper or high gradepaper, the amount and number of fillers and additives are oftenincreased compared to papers of lower quality. While the performancechemicals are added to the suspension or to the paper web to improve therunnability of the papermaking machine and/or increase the quality, theymay influence the performance of each other.

Commonly, papers of higher grade, such as white fine paper, need somedegree of resistance to wetting and penetration of aqueous liquids.Therefore, sizing agents, such as cellulose-reactive sizing agents, aretypically added to the cellulosic suspension and/or the paper web.Cellulose-reactive sizing agents including ketene dimer based sizingagents and acid anhydrides are frequently used in neutral to slightlyalkaline stocks. Acid anhydrides are often used in papermaking processesrequiring fast curing of the sizing agent. Fast curing, i.e. high rateof obtaining hydrophobic properties of the paper, is often required forcoated paper grades. Furthermore, paper sized with acid anhydridesachieves better runnability in printing processes e.g. with respect toreduced tendency of agglomeration of reactive sizing agent relatedderivatives in the printing machinery compared to e.g. paper sized withketene dimer agents. Yet, when acid anhydrides are used in a papermakingprocess where performance chemicals having an impact on the aestheticproperties of the formed paper, such as color and whiteness, are presentin the suspension, uneven distribution of dyes in the finished paper mayoccur which is perceived as color spots.

US 2003/0188393 A1 refers to a process for controlling the brightness ofpaper while using shading dyes.

WO 2007/073321 relates to an aqueous dispersion of cellulose-reactivesizing agent, a method for preparing the dispersion, and a process forproducing paper comprising adding the dispersion to the suspension.

However, an irregular distribution of pigments (dyes), perceived by thehuman eye as colored spots, may be observed in white paper, such aswhite fine paper, when shading dyes are present in the cellulosicsuspension and acid anhydrides are used as a sizing agent. The presenceof colored spots in the paper may occur during initial production.However, colored spots tend to increase over time. Shading dyes aretypically added to a cellulosic suspension in order to increase thewhiteness. Accordingly, it is an objective with the present invention toreduce or eliminate the presence of colored spots in paper. A furtherobject is to reduce or eliminate the presence of colored spots in paperobtained from a process for production of paper comprising adding anacid anhydride to a cellulosic suspension comprising at least oneshading dye. A still further object is to reduce or eliminate irregulardistribution of shading dye in paper obtained from a process forproduction of paper comprising adding an acid anhydride to a cellulosicsuspension comprising at least one shading dye. Further objects willappear hereinafter.

THE INVENTION

The present invention relates to a method for the production of paper,paper which may be obtained by the method, and the use of anitrogen-containing organic compound as further defined herein.

More specifically, the present invention relates to a method forproduction of paper web from an aqueous suspension containing cellulosicfibers comprising adding to the suspension and/or to a paper webprepared from the suspension i) a nitrogen-containing organic compound,ii) an aqueous dispersion of an acid anhydride; and iii) at least oneshading dye, draining the suspension on a wire to form a paper. Theinvention further pertains to the use of a nitrogen-containing organiccompound being an amine or quaternary ammonium compound thereof having amolar mass less than about 180 g/mol in a method for producing paperfrom a suspension as an additive for improving the distribution of dyesin the paper, wherein the suspension comprises at least one shading dye.According to one embodiment of the invention the nitrogen-containingorganic compound is comprised in the aqueous dispersion comprising anacid anhydride.

Also, the present invention pertains to paper obtainable from the methodas disclosed herein, and paper comprising acid anhydride and/orderivatives thereof, a nitrogen-containing organic compound and at leastone shading dye, wherein the paper has no visual agglomerations ofpigments.

Shading dyes, are believed to be more evenly distributed in the paperwhen a nitrogen-containing organic compound is added to a cellulosicsuspension comprising at least one shading dye and sized by the additionof an aqueous dispersion of an acid anhydride.

It has been observed that acid anhydride sizing of cellulosicsuspensions comprising shading dyes used for the manufacturing of paperhas resulted in paper containing agglomeration of pigment renderingvisual spots of color, often of bluish color. Surprisingly, the additionof a nitrogen-containing organic compound counteracts the agglomerationof dyes in the formed paper. What is more, agglomeration of whitepigments is also reduced by the addition of a nitrogen-containingorganic compound.

As used herein, by paper is meant paper intended for printing, such asoffice paper and paper for graphic production, commonly simply referredto as printing paper. Usually, the paper is white, nevertheless, theterm “paper” may include any kind of colored papers. The presentinvention is, however, advantageous for the production of white paper,including white printing paper. According to one embodiment, the paper,suitably white paper, has a grammage up to about 350 g/m². According toone embodiment, the term “paper” does not embrace board and paper board.Yet, paper according to the present invention can be applied as the toplayer of board and paper board.

As whiteness is an issue of the paper manufactured according to thepresent invention, cellulosic fibers, often referred to as pulp, shouldpreferably have a κ (kappa number) of less than about 8 (ISO 302), lessthan about 4, less than about 3, or less than about 2. The cellulosicfibers may be obtained by any pulping processes as long as the fiberscan be used for the production of paper. Pulping processes includemechanical, thermal, semi-mechanical, semi-chemical, and chemicalpulping processes. Commonly chemical processes are sulfite and sulphateprocesses, the latter also referred to as the kraft pulping process.Cellulosic fibers obtained by chemically pulping processes, suitablyfibers from the kraft process, are preferred. The cellulosic fibers areusually also bleached in order to reduce the content of lignin and toachieve a high brightness of the fibers. In order to obtain cellulosicfibers with a high brightness and low content of lignin, raw fibermaterial is commonly subjected to a chemical (kraft) pulping process andsubsequently bleached. The bleaching sequence usually comprises severalbleaching operations interrupted by washing stages and alkalineextraction. Bleaching chemicals include e.g. chlorine dioxide, peroxidessuch as hydrogen peroxide, oxygen, ozone, and peracetic acid. Preferredcellulosic fibers are chemically delignified fibers subsequentlybleached using an elemental chlorine free (ECF) bleaching sequence or atotal chlorine free (TCF) bleaching sequence.

According to one embodiment, the cellulosic fibers used in the presentinvention have a brightness of above about 83 (ISO 3688), above about88, or above about 90. Cellulosic fibers may have a κ (kappa number) ofless than about 4 and a brightness above about 88.

Fiber raw material used in the above pulping processes may include butare not limited to wood such as hardwood, softwood, agriculturalresidues e.g. straw, annual plants (hemp, jute, kenaf, bamboo, etc),recycled or secondary fibers, and any type of deinked fibers.

In order to further improve the quality of the paper with respect toe.g. whiteness, brightness, several non-fiber constituents can be addedto the cellulosic suspension and/or to the dewatered paper web. Thecellulosic suspension and finished paper can contain mineral fillerssuch as kaolin, china clay, titanium dioxide, gypsum, talc, calciumcarbonate e.g. chalk, ground marble and precipitated carbonate.According to one embodiment, the cellulosic suspension contains morethan about 5% by weight of mineral fillers based on dry cellulosicsuspension and optional fillers, such as more than about 10% by weight.

According to the present invention at least one shading dye is presentin the cellulosic suspension. Shading dyes, are compounds which furtherimprove the whiteness of the paper. Whiteness is the sensation of thosecolors perceived by the human eye as being white. In physical terms asubstrate is perceived as white if all visible light (to the human eye)impinging on the substrate is reflected without any loss of intensity.By visible light is meant the part of the electromagnetic spectrum whichcan be detected by the human eye. Very few substrates and chemicalcompounds reflect the entire spectrum of light without absorbing certainwavelengths of the visible electromagnetic radiation. Clouds and newfallen snow reflect most of the light, also some pigments, notablytitanium dioxide, have such a property. However, usually some light isabsorbed by the object, commonly light in the blue range of thespectrum, rendering a yellowish appearance. In order to improve thewhiteness of paper, e.g. decrease of yellowness, shading dyes may beadded in the production process. Fluorescent whitening agents (FWA),including optical brightening agents (OBA) can also be added to thecellulosic suspension, improving inter alia the brightness of the paper.OBA and FWA are fluorescent molecules improving the brightness of paperthrough an additive color mixing process since they emit most of theabsorbed light. Shading dyes, on the other hand, are compounds typicallyincreasing the whiteness by subtracting color mixing.

As used herein, shading dyes are the group of compounds enhancing theperceived whiteness of paper, however, without including brighteningagent, i.e. compounds emitting radiation (fluorescent compounds).

Shading dyes may be selected from naturally and synthetic inorganiccompounds and synthetic organic compounds. One preferred group ofcompounds are synthetic organic compounds. Suitably, the structure ofthe synthetic organic compounds comprises a system of conjugated doublebonds. Said conjugate double bonds can oscillate more or less freely.Typically, the synthetic organic compounds comprise one or more of sameor different chromophoric groups (electron acceptors groups) such asethylene-, keto-, thioketo-, azo-, and carbon-nitrogen-groups.

Compounds improving the whiteness of paper may be selected from thegroup consisting of azo dyes, anthraquinone dyes, indigoid dyes,polymethine dyes, aryl-carbonium dyes, phtalocyanine dyes and nitrodyes.

Azo-group containing compounds usually contain the moiety A-N═N-D, whereA and D often are aromatic moieties. The A-N═N-D moiety may be part of aring system, e.g. hydrazones, or the azo-group containing compound mayform metal complexes. The azo-containing compound may have up to fourazo-groups.

Synthetic organic compounds comprising keto groups include compoundshaving a anthraquinone moiety, such as 9,10-quinone. 9,10-quinone may besubstituted by electron donator groups in the free positions 1 to 4 and5 to 8, specifically in the positions 1, 4, 5, and 8.

Indigoid compounds include the structure element —CO—CX═CY—CO—, where X,Y is selected from O, S, Se, or NH. Indigo (1) and substituted indigocompounds are examples of indigoid compounds.

Polymethine compounds comprise the general structure (2). Polymethinecompounds may be cationic, neutral or anionic, depending on thecharacter of groups A and B.

[A=CR—(X═Y)_(x)—B]^(Z) (2), wherein X and Y independently can be C or N,x is an integer, and z being +n, 0 or −n (n being an integer). Cyanines(Cl basic red 12) and hemicyanines (Cl basic violet 7) are examples ofcationic polymethines where both X and Y are carbon atoms.Diazahemicyanines (Cl basic blue 41), azacarbocyanines (Cl basic yellow11) and diazacarbocyanines (Cl basic yellow 28) are examples ofpolymethines where X and Y are nitrogen or a combination of nitrogen andcarbon. Neutral polymethine compounds may be exemplified by meroxyanineand anionic polymethines by oxonol.

Aryl-carbonium compounds include a group of compounds comprising thegeneral structure (3),

wherein m and n is 0 or 1, X is C or N, Y is O, S or NR, A and B areindependently O, S or NR, and R is an alkyl- or aryl-group.

Other compounds improving the whiteness of paper may includephthalocyanine compounds and nitro containing compounds. The lattertypically comprises two or more aromatic rings, usually benzene ornaphthalene, containing at least one nitro-group and an electron donorgroup such as NH₂ or OH.

Shading dyes are dyes which absorb light (electromagnetic radiation) inthe visible spectrum without significantly emitting electromagneticradiation or without emitting electromagnetic radiation in addition tothe reflected radiation. Accordingly, shading dyes do not increase theintensity of the reflected radiation in the visible spectrum of thesubstrate, i.e. they do not emit radiation (light) or they do notsignificantly emit radiation. Shading dyes can, thus, be referred to assubtractive dyes. Shading dyes (subtracting dyes) absorb thecomplementary wavelength region to the wavelength region leading toyellowness. Shading dyes may be defined as non-fluorescent compounds(dyes) absorbing radiation in the visible spectrum in such a way thatthe perceived whiteness of the paper is increased. Shading dyestypically absorb light in the bluish portion of the visible spectrum(from about 380 nm to about 495 nm), and/or in the orange-reddishportion of the visible spectrum (from about 590 nm to about 750 nm).Shading dyes may be referred to as organic molecules comprisingconjugated systems, i.e. conjugated organic molecules, absorbingelectromagnetic radiation in the visible spectrum, i.e. from about 380nm up to about 750 nm, preferably in the range of from about 380 nm toabout 500 nm and/or in the range of from about 580 nm to about 750 nm).As used here a conjugated system denotes a delocalisation of electronsacross adjacent parallel aligned p orbitals of atoms.

A preferred class of dyes is dyes under the name Viscofil®. Anotherpreferred group of shading dyes are phthalocyanine compounds (organiccompounds comprising a phthalocyanine moiety) including but not limitedto metal phthalocyanine compounds optionally comprising solubilisinggroups such as sulphonic acid functions, e.g. salts ofphthalocyanine-sulfonic acids such as sodium or ammonium salts of copperphthalocyanine. Organic amine salts of sulfonic acids may also becomprised in the phthalocyanine type compounds. Phthalocyanine typecompounds are able to co-ordinate hydrogen and/or metal cations in thecentre. Common coordinated metal cations are copper and cobalt. Otherphthalocyanine dyes include derivatives of metal phthalocyanine andorganic amines. Yet further preferred classes of dyes aretriphenodioxazines commonly referred to as dioxazines. Preferreddioxazines include dihalide triphenodioxazine such as9,10-dichlorotriphenodioxazine and triphenodioxazines comprising aacetylamino or bezoylamino group, ethoxy group, halide atom or anHNCOCH₃ group. Preferred shading dyes are also selected from the groupconsisting of dioxazines and organic compounds comprising aphthalocyanine moiety.

Examples of shading dyes are Viscofil® dyes from Clariant® and Levanyl®dyes from Laxness®, including but not limited to Viscofil®: Orange GG,Orange S-RL, Red R 30, Red BL, Red F5RK, Bordo BB, Violet BLN, Blue B2G,Blue BLF, Levanyl® Violet 23. Further shading dyes are Cartasol® F,Cartasol® K, Cartasol® M, Carta® dyes, Carta® powder dyes, Cartazine®,Diresul® P, Cartaren®, Flexonyl®, Cartacrom®, Hostatint®.

According to one embodiment of the present invention, the cellulosicsuspension may also contain at least one fluorescent compound e.g. OBAs,such as fluorescent organic compounds absorbing ultraviolet light (e.g.from daylight at 300-430 nm) and emitting most of the absorbed energy asblue fluorescent light in the range of from about 400 up to about 500nm. Examples of OBAs are those OBAs sold under the name Leucophor®.

The shading dyes and OBAs may be added to the cellulosic suspensionand/or applied to the surface of a cellulosic sheet or web of paper.They may be added at any point to the cellulosic suspension startingfrom the mixing chest up to and including the head box and/or to theformed web of paper after the head box. The sequence of addition of theshading dye(s) and/or the OBAs, dispersion of acid anhydride, andnitrogen-containing organic compound, where appropriate, the sequence ofthe addition of the shading dyes and/or the OBAs and the acid anhydridedispersion comprising a nitrogen-containing organic compound may varyand may be in any order.

The total amount of added shading dye(s) is usually up to about 400 g/tbased on dry cellulosic suspension and optional fillers, suitably lessthan about 300 g/t. The shading dye(s) may be added to the mixing chest.OBAs, if present in the formed paper, are typically added to thesuspension in an amount of up to about 30 kg/t based on dry cellulosicsuspension, suitably up to about 20 kg/t. Commonly, OBAs are added tothe cellulose suspension up to and including the level box. If bothshading dyes and OBAs are used, the shading dyes may be added prior tothe addition of the OBAs. Shading dyes may e.g. be added to the mixingchest and OBAs to the level box.

The nitrogen-containing organic compound may be selected from aminessuch as primary, secondary and tertiary amines; and quaternary ammoniumcompounds thereof. Suitable nitrogen-containing organic compoundsfurther include monoamines, diamines and polyamines and quaternaryammonium compounds thereof. Suitable quaternary ammonium compoundsinclude protonated, alkylated, arylated and alkarylated amines of theabove-mentioned types, which can be formed by reaction of the amineswith, for example, acids, e.g. hydrochloric acid, and methyl chloride,dimethyl sulphate and benzyl chloride. According to one embodiment, thenitrogen-containing organic compound is an amine or quaternary ammoniumthereof optionally having one or more hydroxyl groups. Preferably, oneor more hydroxyl groups are present in a terminal position of one ormore substituents of the nitrogen-containing compound, i.e. a hydroxylgroup terminated amine or quaternary ammonium compound thereof.

Examples of suitable nitrogen-containing organic compounds include thefollowing amines and their quaternary ammoniums: diethylene triamine,methylene tetramine, hexamethylene diamine, diethyl amine, dipropylamine, di-isopropyl amine, cyclohexylamine, pyrrolidine, guanidine,triethanol amine, monoethanol amine, diethanol amine, 2-methoxyethylamine, aminoethylethanol amine, alanine and lysine. Further examples ofsuitable nitrogen-containing organic compounds include cholinehydroxide, tetramethyl ammoniumhydroxide, tetraethyl ammoniumhydroxide.Preferred nitrogen-containing organic compounds include triethanol amineand quaternary ammonium compounds thereof.

The molar mass of the nitrogen containing organic compound can varywithin wide limits. As used herein, nitrogen-containing organiccompounds refer to non-polymeric compounds, i.e. compounds notcomprising repetitive structural units. Typically, the molar mass of thenitrogen-containing organic compound is up to about 500 g/mol, e.g. upto about 400 g/mol, or up to about 180 g/mol.

In one embodiment of the invention, the molar mass of the amine orquaternary ammonium compound thereof is less than about 180 g/mol, e.g.up to about 170 g/mol or up to about 160 g/mol. The molar mass isusually at least about 30 g/mol. As stated herein, the molar mass of aquaternary ammonium of an amine means the molar mass of the cationicpart of the quaternary ammonium compound, meaning that the anionic partof the quaternary ammonium compound is not included in the molar massgiven above. For nitrogen-containing organic compounds which areselected from amines and quaternary ammoniums thereof having one or morehydroxyl groups, the molar masses may be higher, e.g. less than about500 g/mol and usually less than about 300 g/mol, although theabove-mentioned molar masses are also suitable for such compounds.

The nitrogen-containing organic compound may be added separately/singlyto the cellulosic suspension or in conjunction with other additives suchas the acid anhydride sizing agent, e.g. comprised in the aqueousdispersion of the acid anhydride. If added singly, thenitrogen-containing organic compound can be added to the aqueouscellulosic suspension and/or to the formed paper web, typically at anypoint starting from the machine chest up to and including the head boxand/or to the paper web after the head box. The amount ofnitrogen-containing organic compound as a single addition can be fromabout 0.0004 up to about 0.1% by weight based on dry cellulosicsuspension and optional fillers, e.g. from about 0.0008 up to about0.01% by weight.

According to one embodiment, the nitrogen-containing organic compound isadded in conjunction with the sizing agent.

According to one embodiment, the nitrogen-containing organic compound iscomprised in an aqueous dispersion comprising the acid anhydride.

According to yet a further embodiment of the invention the aqueousdispersion comprising an acid anhydride and a nitrogen-containingorganic compound additionally comprises an anionic polyelectrolyte.

The anionic polyelectrolyte according to the invention can be selectedfrom organic and inorganic compounds and it can be derived from naturalor synthetic sources. The anionic polyelectrolyte has two or moreanionic groups which can be of the same or different types. Examples ofsuitable anionic groups, i.e. groups that are anionic or renderedanionic in an aqueous phase, include silanol, aluminosilicate,phosphate, phosphonate, sulphate, sulphonate, sulphonic and carboxylicacid groups as well as salts thereof, usually ammonium or alkali metal(generally sodium) salts. The anionic polyelectrolytes may bewater-soluble, e.g. linear and branched anionic polyelectrolytes, orwater-dispersable, e.g. cross-linked and/or particulate anionicpolyelectrolytes. According to one embodiment, the water-dispersable andparticulate anionic polyelectrolytes are colloidal, i.e. in thecolloidal range of particle size. The colloidal particles suitably havea particle size from 1 to 100, e.g. from 2 to 70 or from 2 to 40 nm. Thewater-dispersable and particulate anionic polyelectrolytes may containaggregated and/or non-aggregated particles.

Examples of suitable organic anionic polyelectrolytes include anionicpolysaccharides like starches, guar gums, celluloses, chitins,chitosans, glycans, galactans, glucans, xanthan gums, mannans, anddextrins. Further examples of suitable organic anionic polyelectrolytesinclude synthetic anionic polymers such as condensation polymers, e.g.polyurethanes and naphthalene-based and melamine-based polymers, e.g.condensated formaldehyde naphthalene sulfonates and polymers based onmelamine-sulfonic acid, and vinyl addition polymers prepared fromethylenically unsaturated monomers including anionic or potentiallyanionic monomers, e.g. acrylic acid, methacylic acid, maleic acid,itaconic acid, crotonic acid, vinylsulfonic acid, sulfonated styrene andphosphates of hydroxyalkyl acrylates and methacrylates, optionallycopolymerized with non-ionic ethylenically unsaturated monomers, e.g.acrylamide, alkyl acrylates, styrene and acrylonitrile as well asderivatives of such monomers, vinyl esters, and the like.

Examples of further suitable organic anionic polyelectrolytes includewater-soluble branched polymers and water-dispersible crosslinkedpolymers obtained by polymerization of a monomer mixture comprising oneor more ethylenically unsaturated anionic or potentially anionicmonomers and, optionally, one or more other ethylenically unsaturatedmonomers, in the presence of one or more polyfunctional crosslinkingagents. The presence of a polyfunctional crosslinking agent in themonomer mixture renders possible preparation of branched polymers,slightly crosslinked polymers and highly crosslinked polymers that arewater-dispersible. Examples of suitable polyfunctional crosslinkingagents include compounds having at least two ethylenically unsaturatedbonds, e.g. N,N-methylene-bis-(meth)acrylamide, polyethyleneglycoldi(meth)acrylate, N-vinyl (meth)acrylamide, divinyl-benzene,triallylammonium salts and N-methylallyl(meth)acrylamide; compoundshaving an ethylenically unsaturated bond and a reactive group, e.g.glycidyl (meth)acrylate, acrolein and methylol(meth)acrylamide; andcompounds having at least two reactive groups, e.g. dialdehydes likeglyoxal, diepoxy compounds and epichlorohydrin.

The organic anionic polyelectrolyte usually has a degree of anionicsubstitution (DSA) from 0.01 to 1.4, e.g. from 0.1 to 1.2 or from 0.2 to1.0. The anionic polyelectrolyte may contain one or more cationic groupsas long as it has an overall anionic charge. The molar mass of theanionic polyelectrolyte can vary within wide ranges; usually the molarmass is above 200 such as above 500, whereas the upper limit is usually10 million such as 2 million g/mol.

Examples of suitable inorganic anionic polyelectrolytes include anionicsiliceous materials, e.g. anionic silica-based materials prepared fromsilicic acid and clays of the smectite type. Usually, these anionicpolyelectrolytes have negative silanol, aluminosilicate or hydroxylgroups. Examples of suitable inorganic anionic polyelectrolytes includepolysilicic acid, polysilicates, polyaluminiumsilicates, colloidalsilica-based particles, e.g. particles of silica, aluminated(aluminium-modified) silica and aluminiumsilicate, polysilicatemicrogels, polyaluminiumsilicate microgels, silica gels and precipitatedsilica, smectite clays, e.g. montmorillonite, bentonite, hectorite,beidelite, nontronite and saponite. Preferred anionic polyelectrolytesinclude silica-based materials, e.g. colloidal silica-based particles.

The acid anhydride can be any acid anhydride-based sizing agent known inthe art. Suitably, the sizing agent is a hydrophobic acid anhydride.Suitable hydrophobic acid anhydrides can be characterized by the generalformula (I) below, wherein R¹ and R² are independently selected fromsaturated or unsaturated hydrocarbon groups which suitably contain from8 to 30 carbon atoms, or R¹ and R² together with the —C—O—C— moiety canform a 5 to 6 membered ring, optionally being further substituted withhydrocarbon groups containing up to 30 carbon atoms.

R¹—(C═O)—O—(C═O)—R²

Examples of suitable acid anhydrides include alkyl and alkenyl succinicanhydrides, e.g. iso-octadecenyl succinic anhydride, iso-octadecylsuccinic anhydride, n-hexadecenyl succinic anhydride, dodecenyl succinicanhydride, decenyl succinic anhydride, octenyl succinic anhydride,tri-isobutenyl succinic anhydride, 1-octyl-2-decenyl-succinic anhydrideand 1-hexyl-2-octenyl-succinic anhydride. Examples of suitable acidanhydrides further include the compounds disclosed in U.S. Pat. Nos.3,102,064; 3,821,069; 3,968,005; 4,040,900; 4,522,686; and Re. 29,960,which are hereby incorporated herein by reference.

The cellulose-reactive sizing agent may contain one or more acidanhydrides, e.g. one or more alkyl and/or alkenyl succinic anhydrides.Usually, the acid anhydride of this invention is liquid at roomtemperature.

The dispersion suitably contains a dispersant, or dispersant system,comprising an anionic polyelectrolyte and a nitrogen-containing organiccompound. When used in combination, these compounds are effective as adispersant for the acid anhydride sizing agent although the anionicpolyelectrolyte and nitrogen-containing organic compound may not beeffective as a dispersant when used singly. Preferably, the dispersionis anionic, i.e. the dispersant, or dispersant system, has an overallanionic charge.

With respect to the embodiment where the nitrogen-containing organiccompound is comprised in the dispersion of the acid anhydride, the acidanhydride may be present in an amount of from about 0.1 to about 50,e.g. from 0.1 to about 30, or from about 1 to about 20% by weight, basedon the weight of the aqueous dispersion. The optional anionicpolyelectrolyte is usually present in an amount of up to about 100% byweight, usually from 0.1 to 15% by weight, suitably from 0.5 to 10, e.g.from 1 to 7% by weight, based on the weight of the acid anhydride. Thenitrogen containing organic compound can be present in an amount of upto 20% by weight, usually from 0.1 to 15, such as from 0.5 to 10, orfrom 1 to 7% by weight, based on the weight of the acid anhydride. Inaddition to the acid anhydride, anionic polyelectrolyte and nitrogencontaining organic compound, optional additional compounds may bepresent in the dispersion. Examples of such compounds include mono-, di-and poly-anionic and non-ionic surfactants and dispersing agents,stabilizers, extenders and preservative agents such as, for example,hydrolyzed acid anhydrides, e.g. hydrolyzed alkyl and alkenyl acidanhydrides as mentioned above, preferably hydrolyzed alkenyl succinicanhydrides, e.g. hydrolyzed acid anhydrides in the form of carboxylicacid and/or carboxylic acid ester derivatives, anionic surfactants likephosphate esters, such as ethoxylated phosphate esters, alkyl sulphates,sulphonates and phosphates, alkylaryl sulphates, sulphonates andphosphates, e.g. sodium lauryl sulphonate and ethoxylated, phosphatedisotridecylalcohol. If present, the content of such additional compoundsin the dispersion can be from 0.1 to 15, e.g. from 1 to 10, or from 2 to7% by weight, based on the weight of the acid anhydride. Water is alsopresent in the dispersion and may constitute the remainder of thedispersion up to 100% by weight.

The aqueous dispersion comprising the nitrogen-containing organiccompound, the acid anhydride and optionally an anionic polyelectrolytecan be produced by forming a mixture containing the acid anhydride,anionic polyelectrolyte and nitrogen-containing organic compound asdefined above and dispersing the mixture in the presence of water. Thecomponents of the dispersion may be admixed in any order but preferablythe anionic polyelectrolyte and the nitrogen-containing organic compoundare mixed and diluted with water to appropriate concentration, and thenthe acid anhydride is dispersed therein. According to an embodiment thedispersion containing the anionic polyelectrolyte, nitrogen-containingorganic compound and acid anhydride is mixed with starch prior to theaddition to the cellulosic suspension and/or to the paper web. Thestarch which is premixed with the acid anhydride dispersion may formpart of the retention system. The mixture may be dispersed by usingsuitable dispersing equipment providing sufficient degree of dispersing,e.g. a static mixer providing relatively low shear forces. The obtaineddispersion contains droplets of acid anhydride usually having a dropletsize of from 0.1 to 10 μm in diameter.

The aqueous dispersion of acid anhydride comprising thenitrogen-containing organic compound may be added to the cellulosicsuspension, i.e. anywhere between and including the machine chest andhead box, or alternatively to the cellulosic web or sheet, e.g. at thesize press. According to one embodiment, the aqueous acid anhydridedispersion is added to the cellulosic suspension prior to the head box.

The amount of acid anhydride sizing agent added to the cellulosicsuspension or applied to the cellulosic sheet or web can be from about0.01 up to about 1% by weight based on dry cellulosic suspension andoptional fillers, such as from about 0.05 up to about 0.5% by weight.

The method and the use of the nitrogen-containing compound, and theaqueous sizing dispersions are also useful in the manufacture of paperfrom an aqueous cellulosic suspension having a high conductivity. Theconductivity of the suspension that is dewatered on the wire can bewithin the range of from 0.3 mS/cm to 10 mS/cm. According to thisinvention, good results can be achieved when the conductivity is atleast 2.0, e.g. at least 3.5 or at least 5.0, such as at least 7.5ms/cm. Conductivity can be measured by standard equipment such as, forexample, a WTW LF 330 instrument supplied by Christian Berner. Thevalues referred to above are suitably determined by measuring theconductivity of the cellulosic suspension that is fed into or present inthe headbox of the paper machine or, alternatively, by measuring theconductivity of white water obtained by dewatering the suspension. Highconductivity levels mean high contents of salts (electrolytes) which canbe derived from the materials used to form the stock, from variousadditives introduced into the stock, from the fresh water supplied tothe process, etc. Further, the content of salts is usually higher inprocesses where white water is extensively recirculated, which may leadto considerable accumulation of salts in the water circulating in theprocess.

The invention also relates to a paper obtainable from the method asdefined herein and to a paper comprising acid anhydride and/orderivatives thereof, a nitrogen-containing organic compound and at leastone shading dye; and optionally at least one optical brightening agent,wherein the paper has no visual agglomerations of pigments.

The paper may be used in any conventional application. However, thepaper can typically be used as printing paper or copying paper or anyother use involving good printability properties of the paper.

The invention is further illustrated by the following examples, which,however, are not intended to limit the same. Parts and % relate to partsby weight and % by weight, respectively, unless otherwise stated.

EXAMPLE 1

Aqueous dispersions of alkenyl succinic anhydride (ASA) were prepared bydispersing 10 g ASA based on an olefin fraction comprisingiso-hexadecenyl and iso octadecenyl succinic anhydride in 190 g watercontaining 200 mg/l calcium chloride. All dispersion were prepared byusing an Osterizer laboratory mixer. The aqueous phase was first addedto the mixer. After addition of the ASA and optional amine containingsilica sol (dispersion no. 2), the compositions were dispersed at highspeed for 2 minutes.

ASA dispersion no. 1 was prepared by dispersing 10 g ASA (EKA SA 420) inthe presence of 190 g water containing 200 mg/l calcium chloride at atemperature of 0° C. (ASA content of 5% by weight).

ASA dispersion no. 2 was prepared by dispersing 10 g ASA (EKA SA 820 SF)in 190 g water containing 200 mg/l calcium chloride at a temperature of12° C. in the presence of 5 g of a silica sol having a content of SiO₂of 8.0% by weight and containing 50 (42.5% as active) % by weight oftriethanol amine (TEA) based on SiO₂ (ASA content of 5% by weight,triethanol amine content of 0.1% by weight, as active 0.085% by weight).

ASA dispersion no. 3 was prepared by dispersing 10 g of 100% hydrolysedASA in 190 g water containing 200 mg/l calcium chloride at a temperatureof 12° C. (ASA content of 5% by weight).

To all of the above prepared ASA dispersions were added 3.3 g of blueshading dye (undiluted Viscofil® blue BLF, Clariant®) and 28.6 g ofviolet shading dye (undiluted Levanyl® Violet BN-LF, Lanxess®). If theASA dispersions (5%) correspond to an addition of 30 kg ASA/t dry fiberthen the addition of shading dyes correspond to an addition of 100 g/t(blue) and 860 g/t (red/violet), respectively. After mixing the shadingdye containing dispersions were allowed to rest for 20 minutes.

EXAMPLE 2

Paper sheets were prepared according to a process in which the shadingdye containing ASA dispersions of example 1 were added to an aqueouscellulosic suspension comprising 80% of softwood fibers and 20% ofhardwood fibers (based on total fibers) having a fiber concentration of5 g/l, 7% of ground calcium carbonate (GCC), a conductivity of 2 mS/cm(by the addition of calcium chloride), a pH of 7.9, and 1.75 kg/toptical brightener agent (OBA) (Leuchophore, UKO). The dispersions wereadded in an amount of 30 kg/t calculated as ASA based on dry cellulosicsuspension. A retention system was used comprising 5 kg/t of cationicstarch having a D.S. of 0.047 (Perlbond 970) and 7 kg/t polyaluminiumchloride (PAC). After addition of the ASA dispersion to the aqueouscellulosic suspension the so formed cellulosic suspension was mixedduring 10 sec. prior to the formation of a sheet using a KCL sheetformer. The sheets were subsequently pressed during 8 minutes at 3.5 barand thereafter dried. In this example, all amounts are based on drycellulosic suspension if not otherwise indicated. Evaluation of thepaper sheets with respect to color spots. Each paper was divided intofour equal parts, each measuring 8×8 cm, which were labelled 1 through4. Seven individuals (people) were given the task to count the number ofblue and white spots in each paper section and record the total. Theaverage number of spots recorded for each sheet is displayed in table 1below.

TABLE 1 Blue spots White spots Sheet sizing system Average Average ASAdispersion no. 1 13 1 ASA dispersion no. 2 0 0 ASA dispersion no. 3 >25>25

EXAMPLE 3

In this example, all amounts are based on dry cellulosic suspension ifnot otherwise indicated. In a full scale trial printing and writingpaper with a basis weight of 100-280 gram/m² was manufactured using apapermaking process comprising a Fourdrinier Bruderhaus with amulti-cylinder drying process. The furnish suspension contained 50%virgin fibers, 15-20% dry broke, 17% dry broke, and 3-12% GCC orprecipitated calcium carbonate (PCC) filler. The pH range of the furnishcomposition was between 6.8 and 7.4. To the wet end of the process thefollowing components were added:

TABLE 2 Wet-end chemicals and dosages: Chemicals Quality Dosage Point ofaddition ASA Eka SA 420 (a) 0.6-0.8 kg/t before screen Eka SA 820SF (b)Starch Cationic 4.2-5.1 kg/t machine chest Filler GCC 3-12% after levelbox PAC 18% 50 l/hr after level box Shading Blue Viscofil ® 50 g/tmixing chest dyes blue BLF Violet, 430 g/t Levanyl ® Violet BN-LF OBABlancophore ® 15 kg/t level box

a) According to Prior Art

The acid anhydride sizing dispersion (Eka ASA 420) was fed into a starchstream prior to addition to the suspension. A nitrogen-containingorganic compound was not present in the dispersion nor in the cellulosicsuspension

b) According to Invention

The acid anhydride sizing agent EKA SA 820SF was emulsified into astream of silica sol having a content of SiO₂ of 8.0% by weight andcontaining 50% (42.5% as active) by weight of triethanol amine (TEA)based on SiO₂ with a weight ratio of 2:1 prior to feeding the obtaineddispersion into a cationic starch stream prior to addition to thesuspension.

While using the EKA ASA 420 sizing dispersion, blue spots visuallyappeared in the paper after less than 5 days.

When using a sizing dispersion comprising Eka SA 820 SF and silica solhaving a content of SiO₂ of 8.0% by weight and containing 50% (42.5% asactive) by weight of triethanol amine (TEA) based on SiO₂ the paper hadno visual blue spots after 125 day of continuous running.

1. A method for production of paper from an aqueous suspensioncontaining cellulosic fibers comprising adding to the suspension and/orto a paper web prepared from the suspension i) a nitrogen-containingorganic compound, ii) an aqueous dispersion of an acid anhydride; andiii) at least one shading dye, draining the suspension on a wire to forma paper.
 2. The method according to claim 1, wherein the shading dyeabsorbs electromagnetic radiation in the visible spectrum withoutsignificantly emitting electromagnetic radiation or without emittingelectromagnetic radiation.
 3. The method according to claim 2, whereinelectromagnetic radiation is absorbed in the spectrum from about 380 nmup to about 750 nm.
 4. The method according to claim 2 or 3, whereinelectromagnetic radiation is absorbed in the spectrum from about 380 nmup to about 495 nm and/or from about 590 nm up to about 750 nm.
 5. Themethod according to any of the preceding claims, wherein the dye isselected from the group consisting of dioxazines and organic compoundscomprising at least a phthalocyanine moiety.
 6. The method according toany of the preceding claims, wherein the nitrogen-containing organiccompound has a molar mass of up to about 500 g/mol.
 7. The methodaccording to any of the preceding claims, wherein thenitrogen-containing organic compound is an amine or a quaternaryammonium compound thereof.
 8. The method according to any of thepreceding claims, wherein the nitrogen-containing organic compound is anamine or quaternary ammonium compound thereof having a molar mass up toabout 180 g/mol.
 9. The method according to any of the preceding claims,wherein the nitrogen-containing organic compound is comprised in theaqueous dispersion comprising an acid anhydride.
 10. The methodaccording to claim 9, wherein the aqueous dispersion comprising an acidanhydride further comprises an anionic polyelectrolyte.
 11. The methodaccording to claim 10, wherein the anionic polyelectrolyte comprisescolloidal silica-based particles.
 12. The method according to claim 11,wherein the colloidal particles have a particle size from about 1 toabout 100 nm.
 13. The method according to any of the preceding claims,wherein the cellulosic fibers have a kappa number of less than about 8.14. The method according to any of the preceding claims, wherein thecellulosic fibers have a brightness of more than about
 83. 15. Themethod according to any of the preceding claims, wherein theconductivity of the cellulosic suspension is at least about 2.0 mS/cm.16. The method according to any of the preceding claims, wherein thesuspension further comprises at least one optical brightening agent. 17.Paper obtainable according to the method as defined by any of thepreceding claims.
 18. Paper comprising acid anhydride and/or derivativesthereof, a nitrogen-containing organic compound and at least one shadingdye, wherein the paper has no visual agglomerations of pigments.
 19. Thepaper according to claim 18, wherein the pigments are bluish.
 20. Use ofa nitrogen-containing organic compound being an amine or quaternaryammonium compound thereof having a molar mass less than about 180 g/molin a method for producing paper from a suspension as an additive forimproving the distribution of dyes in the paper, wherein the suspensioncomprises at least one shading dye.
 21. The use according to claim 20,wherein the nitrogen-containing organic compound is comprised in anaqueous dispersion comprising an acid anhydride.